I would concentrate on the Hofburg Complex--Hofburg palace, treasury, imperial silver collection, national library (stunning!!) and Michaelskirche. If you'd like to see a museum and felt you had time, I'd do the Albertina which had been a palace as well. I would also go to St. Stephens and Peterskirche. I think all that is doable (churches are open in the evening). If you still had time consider a stroll around the rathuis, parlament and opera house. I also think that a heurigen near the Vienna Woods would be a great idea if you like wine and have the time. I only have come from the airport (I've left town by train) I've done the bus, and I've also used the train to the metro. The train/metro was cheaper and came closer to my final destination, and seemed to come more often, but that may have been luck. Schonbrunn takes about 30 minutes to get to, each way, its awesome, but you get may bang for your travel time by staying inside the ring strasse.